This invention relates to linear electrical motors and more particularly to the frequency control of a linear electrical motor of a linear refrigerant compressor.
In simplistic terms, a linear refrigerant compressor includes an armature mounted between two springs which drives the piston of a refrigerant compressor. The armature is driven backwards and forwards by an electrical driver alternately compressing one or other of the springs.
There is a natural resonant frequency of such linear compressors which is a function of the mass of the armature and the tension of the springs. Because the piston is attached to the armature, the resonant frequency will be affected by the load that is applied to the piston. In most applications, this load is not constant and therefore the resonant frequency of the compressor will not be constant.
To achieve high efficiency, the linear motor should be driven at the resonant frequency of the compressor that is to say, the driver frequency should be as close as possible to the resonant frequency of the linear motor compressor.
There are different prior art methods which attempt to synchronise the driver frequency to the resonant frequency of the compressor. One prior art method measures the pressures on the high pressure side and the low pressure side of the compressor and the driver frequency is adjusted according to these pressure measurements. This method has the disadvantage of not taking into account inherent variations in the initial resonant frequency of linear motor compressors that arise through manufacturing processors.
Another prior art method which attempts to synchronise the driver frequency to the resonant frequency sensors the current wave form and adjusts the driver frequency according to the sensed wave form. The disadvantage of this method is that the relationship between the current wave form and the movement of the armature is not constant for all operating conditions of the compressor.
Australian Patent No 687,294 describes a linear refrigerant compressor having a one-sided armature driver. Back EMF generated by the armature is used to adjust the frequency of the electrical driver. This is achieved by applying power to the compressor when the back EMF is zero which can be measured after no power is applied to the downward cycle of the compressor (i.e. the compressor is powered only on the upward stroke). This prior art system has several disadvantages. For example, the ripple current on the compressor and driver is very high as twice as much current is applied on the upward stroke compared to a double-sided driver. Furthermore, compressor efficiency cannot be maximised as power is applied only in one direction and because the on time to the compressor is constant and the frequency adjusted, the efficiency cannot be maximised over the whole range of working conditions.
It is an object of this invention to provide a method of driving a linear motor compressor at a frequency that will be as close as to the resonant frequency of the compressor by measuring the back EMF generated by the armature of the motor.
Linear electrical motors, as do any other electric motor, produce back EMF proportional to the speed of armature movement. At both ends of the stroke, the armature speed is zero so that the back EMF produced by the linear motor is also zero at both ends of the stroke.
As zero EMF occurs at the point where the direction of movement of the armature is reversedxe2x80x94that is at the beginning of a new cycle or half cycle of the driver waveform, the frequency of the electrical driver can be adjusted so that the change of direction of armature movement will be at the same time as the change of the driving wave from one polarity to the other.
According to one aspect of the invention there is provided a method of controlling the frequency of a driver circuit of a linear electrical motor which drives a linear refrigerant compressor that has a characteristic resonant frequency, said method comprising the steps of measuring the magnitude and polarity of the back EMF at either the start or the end of the stroke of the compressor, analysing the measured back EMF to determine whether the driver frequency is higher or lower than the resonant frequency of the compressor, and adjusting the frequency of the driver to or closer to the resonant frequency of the compressor.
According to another aspect of the invention there is provided a method of operating the driver circuit of a linear electrical motor which drives a linear refrigerant compressor that has a characteristic resonant frequency, said method comprising monitoring the polarity of the back EMF at either the start or the end of the stroke of the compressor, analysing the monitored back EMF and adjusting the frequency of the driver to or closer to the resonant frequency of the compressor.
According to a further aspect of the invention there is provided a control circuit for a linear electrical motor which drives a linear refrigerant compressor that has a resonant frequency, said control circuit comprising means for measuring the magnitude and polarity of the back EMF of the electrical motor at the beginning and/or end of the stroke of the compressor, means for analysing the measured back EMF to determine whether the driver frequency is higher or lower than the resonant frequency of the compressor and means for adjusting the frequency of the driver to or closer to the resonant frequency of the compressor.